1. Field of the Invention
The present invention relates to a thin-film piezoelectric resonator and a method of making the same. The present invention also relates to a band-pass filter utilizing a thin-film piezoelectric resonator (called xe2x80x9cpiezo-resonatorxe2x80x9d below).
2. Description of the Related Art
With the rapid spread of mobile telecommunications equipment such as portable telephones, small and light band-pass filters, as well as resonators needed to make such filters, are in great demand. As known in the art, thin-film piezo-resonators are suitable for producing high-power filters.
A typical thin-film piezo-resonator includes a substrate and a resonator assembly mounted on the substrate. The resonator assembly includes a piezoelectric film and a pair of thin electrodes sandwiching the piezoelectric film from above and below. The substrate is formed with a cavity below the lower electrode of the resonator assembly.
When an AC voltage is applied to the upper and the lower electrode of the piezo-resonator, the sandwiched piezoelectric film vibrates in its thickness direction (which is known as the inverse piezoelectric effect). On the other hand, by the direct piezoelectric effect, a mechanical vibration or elastic wave is converted into a corresponding electrical signal. The elastic wave is a longitudinal wave whose main displacement occurs in the thickness direction of the piezoelectric film. In such a thin-film piezo-resonator, the resonator assembly resonates when its thickness H is equal to n/2 times the wavelength of the elastic wave (where n is an integer). Supposing that the propagation velocity of the elastic wave is V (which depends on the material used), the resonance frequency F is expressed by a formula F=nV/2H. This means that a piezo-resonator having desired frequency characteristics can be obtained by adjusting the thickness H of the resonator assembly. Further, by connecting such resonators in a ladder configuration, it is possible to produce a band-pass filter which allows only those electric waves lying within a certain frequency range to pass.
In the above-described thin-film piezo-resonator, desired resonance characteristics are attained by providing a cavity or hole below the lower electrode. Techniques suitable to making such a cavity are disclosed in xe2x80x9cZnO/SiO2-DIAPHRAGM COMPOSITE RESONATOR ON A SILICON WAFERxe2x80x9d (K. NAKAMURA, ELECTRONICS LETTERS Jul. 9, 1981 Vol. 17 No. 14 p507-509), JP-A-60 (1985)-189307, JP-A-2000-69594, U.S. Pat. No. 6,060,818 and U.S. Pat. No. 5,587,620 for example.
FIG. 20 shows, in section, a thin-film piezo-resonator disclosed in the above-mentioned xe2x80x9cZnO/SiO2xe2x80x94 DIAPHRAGM COMPOSITE RESONATOR ON A SILICON WAFERxe2x80x9d. The thin-film piezo-resonator (generally indicated by reference numeral 700) includes a (100)-cut silicon substrate 710 and a resonator assembly 720 supported by the substrate 710. The resonator assembly 720 is made up of a lower electrode 721, a piezoelectric film 722, and an upper electrode 723. The silicon substrate 710 has an upper surface upon which a SiO2 film 711 is formed by thermal oxidation. The resonator assembly 720 is placed directly on the SiO2 film 711. The silicon substrate 710 is formed with a cavity 710a whose upper opening is closed by the SiO2 film 711. The cavity 710a can be produced by anisotropic etching with respect to the (100) surface of the silicon substrate. The anisotropic etching is performed from below the silicon substrate 710 with the use of KOH solution or EDP solution (ethylenediamine+pyrocatechol+water) for example.
The above anisotropic etching relies upon the fact that the etching rate with respect to the (100) surface of the substrate 710 is significantly greater than the etching rate with respect to the (111) surface. Therefore, the resonator assembly is to be provided only on the (100) surface of the substrate 710. However, such positional limitation is disadvantageous since it restricts the option of the material suitable for making the components of the resonator assembly 720, while also depriving the resonator assembly components of freedom of orientation. Another disadvantage is that the lower opening of the cavity 710a tends to be unduly large due to the nonupright side wall 710axe2x80x2 of the cavity 710a. In the illustrated arrangement, the side wall 710axe2x80x2 corresponds to the (111) surface of the substrate 710 and has an inclination of 54.7 degrees with respect to the (100) surface of the silicon substrate 710. Due to this, the cavity 710a has a large opening in the bottom surface of the silicon substrate 710. For instance, when the substrate 710 has a thickness of 300 xcexcm, the lower opening of the cavity 710a is larger than the upper opening by more than 420 xcexcm. Unfavorably, such a large opening of the cavity 710a reduces the mechanical strength of the piezo-resonator 700. In addition, it contributes to an increase in the overall size of the piezo-resonator 700. With the use of such oversize piezo-resonators, a compact band-pass filter cannot be obtained. Specifically, when the thickness of the substrate 710 is 300 xcexcm, the lower opening of the cavity 710a is larger than the upper opening by more than 420 xcexcm, as noted above. Thus, the distance between the neighboring upper openings should be more than 420 xcexcm. Further, as the distance between the adjacent upper openings increases, the length of the wiring pattern for connecting the adjacent resonator assemblies should also increase. This leads to an increase in the resistance of the wiring pattern. A greater resistance of the wiring pattern can be a major factor that prevents the improvement of the filter characteristics in a high-frequency band.
FIG. 21 shows a thin-film piezo-resonator disclosed in JP-A-60-189307. The piezo-resonator 800 includes a substrate 810, and a resonator assembly 820 which is made up of a lower electrode 821, a piezoelectric film 822, and an upper electrode 823. A cavity 830 is provided between the substrate 810 and the resonator assembly 820. According to the Japanese document, the piezo-resonator 800 is fabricated in the following manner. First, a sacrifice layer for the cavity 830 is formed in a pattern on the substrate 810. Next, a SiO2 film 840 is formed on the sacrifice layer 840 in a manner such that part of the sacrifice layer is exposed. Then, the resonator assembly 820 is provided on the SiO2 film 840. Finally, the sacrifice layer is removed by wet etching, so that the cavity 830 appears below the resonator assembly 820. According to this method, the cavity 830 is kept from becoming too large with respect to the resonator assembly 820.
In the thin-film piezo-resonator utilizing a longitudinal vibration in the thickness direction, a high-orientation piezoelectric film is required to provide excellent resonance characteristics. According to the technique disclosed in JP-A-60-189303, however, it is difficult to give a sufficiently high orientation to the piezoelectric film 822. The cavity 830 below the resonator assembly 820 has a length L15, which needs to be at least a few micron meters when a twist and oscillation displacement of the resonator assembly 820 are taken into consideration. Unfavorably, the sacrifice layer, formed to have a thickness corresponding to the length L15, has a greater surface roughness than that of the silicon substrate 810. This deteriorates the orientation of the lower electrode 821 and the piezoelectric film 822 both of which are grown on the sacrifice layer via the SiO2 film 840. As a result, it is difficult to obtain good resonance characteristics with the thin-film piezo-resonator.
FIG. 22 is a sectional view of a thin-film piezo-resonator disclosed in JP-A-2000-69594. The thin-film piezo-resonator 900 includes a silicon substrate 910, and a resonator assembly 920 made up of a lower electrode 921, a piezoelectric film 922 and an upper electrode 923. A cavity 910a is provided below the resonator assembly 920. According to this document, to make the thin-film piezo-resonator 900, the silicon substrate 910 is etched to form a recess serving as the cavity 910a. Then, a SiO2 film 930 is formed by thermal oxidation on a surface of the silicon substrate 910. Next, material is deposited in the cavity 910a to form a sacrifice layer. After the depositing, the surface of the sacrifice layer is polished and cleaned. Next, the resonator assembly 920 is formed on the sacrifice layer in a manner such that the sacrifice layer is partially exposed. Finally, the sacrifice layer is removed by wet etching.
However, the method disclosed in JP-A-2000-69594 includes a large number of steps such as the step of depositing the sacrifice layer in the cavity 910a, the step of polishing the sacrifice layer and so on. Therefore, it is difficult to manufacture the thin-film piezo-resonator at a low cost and at a high yield.
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to solve or reduce the above conventional problems, and to provide a thin-film piezo-resonator suitable for miniaturization and having a highly oriented piezoelectric film, to provide a band-pass filter including this thin-film piezo-resonator, and to provide a method of making such a thin-film piezo-resonator.
According to a first aspect of the present invention, there is provided a thin-film piezo-resonator comprising: a substrate having a first surface and a second surface opposite to the first surface, the substrate being formed with a cavity that has a first opening in the first surface and a second opening in the second surface; and a resonator assembly including an exciter composed of a first electrode contacting the first surface, a piezoelectric layer on the first electrode and a second electrode on the piezoelectric layer, the assembly being disposed at a location corresponding to the cavity. The cavity includes a side surface extending in a substantially perpendicular direction to the first surface of the substrate.
In this specification, the xe2x80x9cexciterxe2x80x9d refers to the overlapping region of the first and the second electrodes (or electrode patterns) and the piezoelectric layer.
With the above arrangements, it is possible to fabricate a thin-film piezoelectric resonator that is compact and exhibits excellent resonance characteristics. The compactness results from the cavity that penetrates the substrate in a non-flaring manner, with an uniformly upright side surface. Such a cavity may be produced by dry etching such as Deep-RIE (Reactive Ion Etching), regardless of the cut condition of the substrate. With the use of such compact resonators, a compact filter can be obtained. Further, since the cut condition of the substrate does not affect the formation of the cavity, the most suitable cut surface can be realized in the substrate. The free selectability of the cut surface facilitates the forming of a highly oriented first electrode (lower electrode) thereon. This allows a highly oriented piezoelectric layer to be formed on the first electrode. Accordingly, it is possible to produce a thin-film piezo-resonator with excellent resonance characteristics.
The first electrode and the second electrode may be formed of e.g. aluminum (Al), copper (Cu), molybdenum (Mo), chromium (Cr), titanium (Ti) and platinum (Pt). The piezoelectric layer may be formed of e.g. aluminum nitride (AlN), zinc oxide (ZnO), lead zirconate titanate (PZT), and lead titanate (PbTi3). The substrate may be made of silicon.
Preferably, the first electrode may include a uniaxially oriented single-layer conductive member or uniaxially oriented multi-layer conductive member. In addition, the piezoelectric layer may also be uniaxially oriented. Preferably, the substrate may be a (111)-cut silicon substrate, so that its first and second surfaces are a (111) surface. These arrangements are preferable in providing a highly oriented piezoelectric layer.
Preferably, the first electrode may include a single conductive layer containing either one of (111)-uniaxially oriented Al and (111)-uniaxially oriented Cu. Or, the first electrode may include a stack of uniaxially oriented conductive layers including a first conductive layer held in contact with said first surface, the first conductive layer containing either one of (111)-uniaxially oriented Al and (111)-uniaxially oriented Cu. Or, the first electrode may have a two-layer structure including a first conductive layer and a second conductive layer, where the first conductive layer, held in contact with the first surface of the substrate, contains either one of (111)-uniaxially oriented Al and (111)-uniaxially oriented Cu, while the second conductive layer contains (110)-uniaxially oriented Mo.
With the above arrangement, it is possible to form a highly oriented first electrode on the (111)-cut silicon substrate.
Preferably, the piezoelectric layer may be made of either one of (002)-uniaxially oriented AlN and (002)-uniaxially oriented ZnO for high orientation.
Preferably, the resonator of the present invention may further include a cover substrate for protection of e.g. the resonator assembly. The cover substrate may be bonded to the second surface of the substrate so as to close the cavity.
Preferably, the first electrode and the piezoelectric layer may each include a portion exposed to the cavity. Such exposure is advantageous to providing the resonator with excellent resonance characteristics.
According to a second aspect of the present invention, there is provided a thin-film piezo-resonator that includes: a (111)-cut silicon substrate; a first electrode formed on the substrate and containing either one of Al and Cu; a piezoelectric layer formed on the first electrode and containing either one of AlN and ZnO; and a second electrode formed on the piezoelectric layer. The silicon substrate includes a first surface which is a (111) surface. The first electrode is held in contact with the first surface of the substrate.
According to a third aspect of the present invention, there is provided a thin-film piezo-resonator that includes: a substrate having a first surface and a second surface opposite to the first surface, the substrate being formed with a cavity that has a first opening in the first surface of the substrate; and a resonator assembly including a first electrode contacting the first surface of the substrate, a piezoelectric layer on the first electrode and a second electrode on the piezoelectric layer. The resonator assembly is disposed at a location corresponding to the cavity. Each of the first electrode and the piezoelectric layer includes a portion exposed to the cavity.
With the above arrangements, the resonator assembly exhibits better resonance characteristics than when it is isolated from the cavity. Further, when the opening of the cavity is wide enough to allow not only the first electrode but also the piezoelectric layer to be exposed, resonance characteristics such as the minimum insertion loss or attenuation pole suppression can be improved.
According to a fourth aspect of the present invention, there is provided a filter that includes: a substrate having a first surface and a second surface opposite to the first surface, where the substrate is formed with a plurality of cavities spaced from each other; a first electrode pattern held in contact with the first surface of the substrate; a piezoelectric layer on the first electrode pattern; a second electrode pattern on the piezoelectric layer; and a plurality of resonator assemblies provided by a combination of the first electrode pattern, the piezoelectric layer and the second electrode pattern, where each of the resonator assemblies corresponds in position to one of the cavities. Each of the cavities has a side surface extending in a substantially perpendicular direction to the first surface of the substrate.
Preferably, each of the cavities may include a first opening in the first surface of the substrate and a second opening in the second surface of the substrate, where the distance between adjoining first openings is no greater than 420 xcexcm.
With the above arrangement, it is possible to provide a compact filter. Further, since the connecting distance between any adjoining resonator assemblies can be short, the resistance of the wiring pattern is also reducible. Advantageously, a filter with a less resistive wiring pattern exhibits better performance in a high-frequency band.
In a conventional filter which includes a silicon substrate formed with several cavities (each corresponding in position to one of the piezoelectric resonators), the upper openings of adjoining cavities should be spaced from each other by more than 420 xcexcm (supposing that the thickness of the substrate is 300 xcexcm or more) due to the downward flare of the cavities (see FIG. 20). According to the present invention, on the other hand, the distance between adjoining first or upper openings is 420 xcexcm or smaller by forming each cavity in a manner such that its side surface extends perpendicularly to the substrate. As a result, the filter of the present invention can be smaller than a conventional filter.
Preferably, the resonator assemblies used for the filter of the present invention may include a first group of resonator assemblies connected in series and a second group of resonator assemblies connected in parallel. This makes the filter a ladder type.
Preferably, the first electrode pattern and the piezoelectric layer may be exposed to one of the cavities.
According to a fifth aspect of the present invention, there is provided a filter that includes: a substrate that has a first surface and a second surface opposite to the first surface and is formed with a plurality of cavities each including a first opening in the first surface of the substrate and a second opening in the second surface of the substrate; a first electrode pattern held in contact with the first surface of the substrate; a piezoelectric layer on the first electrode pattern; a second electrode pattern on the piezoelectric layer; and a plurality of exciters provided by the combination of the first electrode pattern, the piezoelectric layer and the second electrode pattern, where each of the exciters corresponds in position to one of the cavities. The first electrode pattern and the piezoelectric layer each include a portion exposed to one of the cavities.
In the above-mentioned aspects of the present invention, the first and the second openings of the cavity may preferably be circular or oval rather than rectangular. This is because a rectangular opening is more difficult to make than a smoothly curved opening by dry-etching, since the etching rate for the corners of the opening tends to be slower than the etching rate for the other portions. In particular, when several openings of different sizes are to be made in a single substrate, the production efficiency is significantly higher in making arcuate openings than in making rectangular openings.
As noted above, the first electrode or the piezoelectric layer may have a portion exposed to a cavity for better resonance characteristics. Preferably, these exposed portions may be made of a material which is not etched by a fluorine gas. Examples of such material are aluminum and copper. With this arrangement, the first electrode and the piezoelectric layer will not or hardly be damaged in performing Deep-RIE.
In this specification, as defined above, an xe2x80x9cexciterxe2x80x9d is the overlapping region of the first and the second electrodes (or electrode patterns) and the piezoelectric layer. More specifically, the first electrode includes a xe2x80x9cfirst exciter portionxe2x80x9d that overlaps the second electrode. Likewise, the second electrode includes a xe2x80x9csecond exciter portionxe2x80x9d that overlaps the first electrode. In symmetry, the first and the second exciter portions have the same or substantially same configuration. The exciter is the assembly of the first and the second exciter portions and a portion of the piezoelectric layer that is sandwiched between the first and the second exciter portions. Since the first and the second exciter portions are (substantially) the same in shape, the desired capacitance is precisely attained between the first and the second exciter portions. Preferably, each exciter portion as a whole or in part may be circular or oval.
In the first to fifth aspects of the present invention, the area of the first opening of a cavity may preferably be 1xcx9c2.25 times larger than the area of the above-defined first or second exciter portion. With this design, the resonator assembly can exhibit good resonance characteristics, while being prevented from suffering deformation or damage.
According to a sixth aspect of the present invention, there is provided a method of making a thin-film piezo-resonator. The method includes the steps of: preparing a substrate including a first surface and a second surface opposite to the first surface; forming a resonator assembly which includes a first electrode held in contact with the first surface of the substrate, a piezoelectric layer formed on the first electrode and a second electrode formed on the piezoelectric layer; and forming a cavity by dry-etching the substrate, where the cavity is disposed at a location corresponding to the resonator assembly, and opened in the first and the second surfaces of the substrate. The cavity includes a side surface extending in a substantially perpendicular direction to the first surface of the substrate.
According to a seventh aspect of the present invention, there is provided a method of making a thin-film piezo-resonator. The method includes the steps of: preparing a substrate including a first surface and a second surface opposite to the first surface; forming a resonator assembly which includes a first electrode held in contact with the first surface of the substrate, a piezoelectric layer formed on the first electrode and a second electrode formed on the piezoelectric layer; and forming a cavity by dry-etching the substrate, where the cavity is disposed at a location corresponding to the resonator assembly, and opened in the first and second surfaces of the substrate. The first electrode and the piezoelectric layer are caused to be partially exposed to the cavity at the cavity-forming step.
In the sixth and the seventh aspects of the present invention, the dry etching may be Deep-RIE. The method may further include the step of bonding a cover substrate to the second surface of the substrate so as to close the cavity. In the method, a groove for dividing the substrate may also be formed by etching at the cavity-forming step.
Other features and advantages of the present invention will become apparent from the detailed description given below with reference to the accompanying drawings.